The use of tapped transformers to control the voltage delivered to a load is widespread in power supply systems where the load may be a network delivering local electric power and the tapped transformer can be provided at a transformer station between the power-generating unit and the local network or between power transmission lines from a power-generating source to the local network.
The tap selector can comprise an electric motor-driven set of movable tap selector contacts which are caused to engage fixed tap selector contacts associated with the taps of a power transformer, and associated drives also operated by that electric motor for synchronously with the tap selection displacement, can operate bypass switch contacts, mechanisms for tripping vacuum-switching cells and even contacts for coarse selection of a tap range for reversing contacts. Reference may be had to the commonly-owned copending application Ser. No. 09/165,494, filed Oct. 2, 1998, now U.S. Pat. No. 6,060,669, corresponding to German application 197 43 864.4 filed Oct. 4, 1997.
In that system the electric motor driven by the tap selector, not only drives the shaft connected with the tap selector contacts, but also drives a shaft connected with the bypass contacts and a shaft operating the vacuum-switching cells for each of the phases of the system to be switched.
A typical operation of such a switch will have, in shifting from a tap n to a tap n+1, a pair of movable selector contacts which, in a previous state both rest upon the fixed contact corresponding to the tap n. Each of these movable contacts may be connected to a switching impedance, e.g. a coil. At the opposite ends, the coils may be bridged by a vacuum-switching cell having a mechanism that can be rapidly tripped to open-circuit that cell and a cam mechanism to close electrically the vacuum-switching cell. Through a pair of bypass contacts, the ends of the impedances bridged by the vacuum-switching cell are connected to the load.
In a typical switching operation, driven by the electric motor, with the two movable contacts of the tap selector on the fixed contact of the preceding tap n, the bypass contact of the impedance associated with the leading selector contact is opened, followed by open-circuiting of the vacuum-switching cell and shifting of the movable contacts of the tap selector to bring the leading movable contact into engagement with the fixed contact of an adjacent transformer step n+1. The vacuum-switching cell is then closed followed by closing of the bypass contact so that the tap selector movable contacts bridge the adjacent fixed contacts of the respective taps. The bypass contact of the upstream tap selector contact can then be opened, followed by open-circuiting of the vacuum-switching cell and the trailing tap selector contact moved onto the fixed contact for tap n+1, whereupon bypass contacts and the vacuum-switching cell are then closed as has been described. With this sequence, an interruption-free switching can be accomplished with a minimum of contact wear and burn off even under load.
In German patent document 42 14 431, a motor drive for such a tap selector has already been described and it has been suggested that information with respect to that motor drive can serve to indicated the position of the tap selector during a tap-selection operation. In fact in this system information is derived with respect to the tap selector setting, the attainment of respective upper and lower end positions, the mode of operation selected and the triggering of the motor protection switch which is generally designed to prevent overloading of the motor. In practice, however, this information has not been found sufficient to provide a satisfactory monitoring function for the operation of the tap selector. In the brochure entitled Microprocessor Controlled Voltage Regulator TCS, publication VK 34/96 DE-0896/1000, issued by Maschinenfabrik Reinhausen GmbH, the present assignee, the microprocessor-controlled voltage regulator is described which can achieve a monitoring function which includes:
monitoring of the circular-blind current between two parallel-switched transformers, PA1 the achievement of over-current blocking for the tap selector, PA1 the under-voltage monitoring, PA1 an over-voltage monitoring with safety shut-off, and PA1 a setpoint and actual value comparison of the controlled voltage. PA1 (a) displacing the tap selector with an electric drive motor and during tap-selection operation of the electric drive motor measuring effective values of voltage applied to and current drawn by the electric drive motor; PA1 (b) automatically calculating true power of the electric drive motor from the effective values of voltage applied to and current drawn by the motor as measured in step (a) and determining a torque developed by the motor from the calculated true power; PA1 (c) simultaneously with the determination of the torque developed by the motor, generating a position value representing an actual setting of the tap selector and thereby forming a corresponding pair of determined values of developed torque and tap-selector setting; PA1 (d) comparing the pairs of determined values with previously stored apparatus-specific setpoint pairs of corresponding values, optionally corrected for ambient temperature; and PA1 (e) upon deviation of a determined value pair from a setpoint value pair exceeding a predetermined threshold difference, generating a signal for inactivating the drive of the tap selector or initiating a maintenance operation thereof. PA1 (a) displacing the tap selector with an electric drive motor and during tap-selection operation of the electric drive motor measuring effective values of voltage applied to and current drawn by the electric drive motor; PA1 (b) automatically calculating true power of the electric drive motor from the effective values of voltage applied to and current drawn by the motor as measured in step (a) and determining a torque developed by the motor from the calculated true power; PA1 (c) simultaneously with the determination of the torque developed by the motor, generating a position value representing an actual setting of the tap selector; PA1 (d) storing values of the determined torque as a function of time; PA1 (e) synchronizing tap selection under load with stored values of determined torque as a function of time by generating at least one synchronization pulse at a characteristic state of tap selection corresponding to a certain point in time (t.sub.syn); PA1 (f) thereafter detecting a normalized course of the torque of the motor in a typical time range (t.sub.0 -t.sub.1, . . . , t.sub.syn -t.sub.n) forming a determined window corresponding to a switch-specific part of the tap-change sequence; PA1 (g) comparing the determined window (t.sub.0 -t.sub.1, . . . , t.sub.syn -t.sub.n) with a stored setpoint value of the window; and PA1 (h) upon deviation of a determined window from the setpoint window exceeding a predetermined threshold difference, generating a signal for inactivating the drive of the tap selector or initiating a maintenance operation thereof.
Even in this system data is not obtained which allows a direct monitoring of the operating state of the tap selector. The input parameters here are only currents and voltages which without additional information and without special information processing, cannot provide a suitable indication of the operating state.
Japanese patent application JP 60 176213 A has described the detection of the torque of the drive shaft which runs from the electric motor drive to the tap selector and the storage of this torque so that the instantaneous torque curve can be compared with a characteristic type-specific setpoint torque curve. A similar process is described in East German patent DD 246 409 in which the torque curve over times is measured in a tap selection operation and the result is compared with a typical torque curve as a function of time for the respective tap selector. Should an impermissible deviation of the actual value from the setpoint value be determined, shutdown will occur. However, because of the difficulty in detecting the torque, these processes have not found significant use in practice.
Mention should also be made of prior proposals whereby the respective torque is not determined directly by mechanical measuring means but rather is detected indirectly by a detection of the efficiency of the drive motor which produces that torque. Even this approach has not found significant use in practice heretofore.